doc: Improve documentation strings in parse-tree directory. * lex.ll (F__display_tokens__): Add seealso reference. * lex.ll (F__token_count__): Add seealso reference. * lex.ll (F__lexer_debug_flag__): Document function. * oct-parse.in.yy (Fautoload): Add additional calling form. Rephrase several sentences. * oct-parse.in.yy (Fmfilename): Make single sentence description stand apart from the rest of documentation. * oct-parse.in.yy (Fsource): Make single sentence description stand apart from the rest of documentation. Add seealso link to 'run'. * oct-parse.in.yy (Fbuiltin): Change type to "Built-in Function" from "Loadable Function". * oct-parse.in.yy (Feval): Rephrase several sentences. Add programming note suggesting the use of alternatives like try/catch or unwind_protect. * oct-parse.in.yy (F__parser_debug_flag__): Document function. * pt-mat.cc (Fstring_fill_char): Use semicolon in place of period for stronger idea linkage.